1. Field of the Invention
The present invention relates to the structure of an axial flow fan such as a propeller fan.
2. Background Information
Axial flow fans such as propeller fans, for example, are commonly used as blowers in outdoor units for air conditioners. FIG. 11 shows an example of the configuration of an outdoor unit for an air conditioner employing an axial flow fan such as a propeller fan as a blower unit.
That is, as shown in FIG. 11, for example, in an outdoor unit for an air conditioner, a blower unit 3 is configured by a propeller fan 4 that is an example of an axial flow fan, a bellmouth 5 that is positioned on the outer peripheral side of the propeller fan 4 and partitions the propeller fan 4 into a rear suction region X of the propeller fan 4 and a front blowout region Y, and a fan guard 6 that is positioned on the blowout side (front side) of the propeller fan 4. The blower unit 3 is disposed inside a box-like body casing 1 on an airflow downstream side of a heat exchanger 2 in a backside air suction opening 10a. Reference numeral 12 is a fan motor that drives the propeller fan 4 to rotate, and the fan motor is supported by and fixed to an unillustrated fan motor attachment bracket disposed so as to be positioned on the downstream side of the heat exchanger 2.
Additionally, the propeller fan 4 is configured by a hub 14 that is coupled and fixed to a drive shaft 12a of the fan motor 12 and which, as shown in enlarged view in FIG. 12 and FIG. 13, for example, serves as the rotational center of the propeller fan 4 and by plural blades 13, 13, 13 that are disposed integrally on the outer peripheral surface of the hub 14.
In the case of an outdoor unit of this structure, there is the drawback that noise during operation becomes higher because of noise from the propeller fan 4 itself and also noise generated as a result of the blowout airflow from the propeller fan 4 colliding with downstream structural objects such as the fan guard 6.
Thus, in order to reduce the total noise when an axial flow fan such as the propeller fan 4 is utilized as a blower in an outdoor unit for an air conditioner as described above, measures and considerations have heretofore been performed, such as optimizing the shape of the wing surfaces of the blade portion of the propeller fan and making airfoil wings having excellent aerodynamic performance, for example. However, the following problems cannot be solved just by these silencing techniques.
That is, in the blade structure of the propeller fan 4 shown in FIG. 13, for example, when the blades 13, 13, 13 rotate, airflows (P) that wind around from pressure surfaces 13d where pressure is high towards negative pressure surfaces 13e where pressure is low occurs on the outer peripheral sides of the blades 13, 13, 13, and wing tip vortexes (Q) such as shown in the drawing are formed by those airflows (P). Additionally, turbulence of the blowout airflows resulting from the wing tip vortexes (Q) occurring due to the airflows (P) that wind around from the blowout side towards the suction side near outer peripheral portions of the blades 13, 13, 13 becomes layered, gradually grows and increases downstream as shown in FIG. 14 and FIG. 15, for example, eventually separates from the negative pressure surfaces 13e of the blades 13, 13, 13, and interferes with the pressure surfaces 13d, 13d of the blades 13, 13 and 13 that are adjacent and the inner peripheral surface of the bellmouth 5 or the fan guard 6 that is a structural object on the downstream side of the blower, which further increases noise.
In particular, as shown in FIG. 15, the wing tip vortexes (Q) that have separated from the negative pressure surfaces 13e of the blades 13, 13, 13 interfere with the adjacent blades 13, 13, 13 as described above, whereby the turbulence becomes even greater and, as a result, causes an even greater amount of noise on the downstream side of the blower.
With respect to this phenomenon, when the wing chord length of the blades 13, 13, 13 is shortened in order to make the blower lightweight (low-cost), the inherent wing row effect of the blades 13, 13, 13 becomes smaller, so as shown in FIG. 16, for example, it becomes easier for the wing tip vortexes (Q) to move even farther from the negative pressure surfaces 13e and interfere with the adjacent blades 13, 13, 13 even earlier than in the above-described case, so it becomes easier for the amount of noise to increase even more.
In relation to this, as shown in FIG. 17, for example, there is an axial flow fan where entire outer peripheral portions 13c from front edges 13a of the blades 13, 13, 13 to rear edges 13b thereof are bent towards the negative pressure sides (suction sides) (by disposing folded portions or bent portions) and where the radial direction width of the bent portions 13c is gradually made larger from the front edges 13a to the rear edges 13b (e.g., Japanese Patent Publication No. 3,629,702).
According to this configuration, as shown in FIG. 18 and FIG. 19, for example, the airflows (P) on the pressure surface 13d sides of the blades 13, 13, 13 wind around into outer peripheral end concave circular arc-shaped negative pressure surfaces 13e of the blades 13, 13, 13 smoothly along outer peripheral end convex circular arc-shaped pressure surfaces 13d of the blades 13, 13, 13, the vortex diameters become small and stable, and the flows of the airflows (R) in the outer peripheral direction of the blades 13, 13, 13 on the negative pressure surfaces 13e no longer interfere with the wing tip vortexes (Q).
Additionally, with respect to this action, when the width of the bent portions 13c of the blade outer peripheral end portions 13c gradually becomes larger from the vicinities of the front edges 13a of the blades 13, 13, 13 towards the vicinities of the rear edges 13b as described above, the blades come to smoothly exhibit an effect from the front edges 13a to the rear edges 13b in correspondence to the vortex diameters of the wing tip vortexes (Q) that gradually develop and whose vortex diameters are enlarged from the front edges 13a of the blades 13 to the rear edges 13b, and it also becomes difficult for the generated wing tip vortexes (Q) to separate from the blade negative pressure surfaces 13e. 
For that reason, even when the wing chord length has been shortened in order to make the blades 13, 13, 13 lightweight, for example, the wing tip vortexes (Q) no longer interfere with each other between the adjacent blades 13, 13, 13, and there is also less turbulence in the blowout airflows on the downstream side of the blower.
As a result, noise when the axial flow fan is incorporated in an outdoor unit for an air conditioner also becomes effectively reduced.
However, in a case where the rear edges 13b of the blades 13 of the fan impeller are surrounded by the bellmouth 5 as mentioned above, leakage flows (S) such as shown in FIG. 20 that pass through the gap between the bellmouth 5 and the outer peripheral portions of the blades and flow out from the pressure surfaces 13d to the negative pressure surfaces 13e exist separately from the wing tip vortexes (Q).
That is, the wing tip vortexes (Q) that leak from the negative pressure surfaces 13e of the blade outer peripheries to the pressure surface 13d as mentioned previously gradually develop from the front edges 13a of the blade outer peripheries towards the rear edges 13b along the negative pressure surfaces 13e of the blades 13. However, in the region where the blades 13 are surrounded by the bellmouth 5, the flows along the negative pressure surfaces 13e suddenly change their trajectories in a direction away from the negative pressure surfaces 13e (see the one-dotted chain lines in FIG. 20).
One reason why the wing tip vortexes (Q) suddenly change their trajectories in this manner is because of the presence of the leakage flows (S) that pass through the gap between the outer peripheral portions of the blades 13 and the bellmouth 5 and flow out from the pressure surfaces 13d of the blades 13 to the negative pressure surfaces 13e. 
The leakage flows (S) arise in the portion surrounded by the bellmouth 5. Additionally, the leakage flows (S) become stronger particularly at the portion of the bellmouth 5 that is straight in the axial direction of the air suction opening in the bellmouth 5, and the turbulence of the flows becomes stronger downstream.
With respect thereto, the turbulence of the flows resulting from such leakage flows (S) cannot be sufficiently reduced simply by disposing the bent portions 13c in the blade outer peripheral portions as in the above-described conventional example. Additionally, the leakage flows (S) eventually cause turbulence at the outer peripheral portions of the rear edges 13b of the negative pressure surfaces 13e and result in a rise in noise.
Moreover, because the leakage flows (S) flow out in the direction of the wing tip vortexes (Q) from the outer peripheral ends of the blades 13, they also play the role of causing the wing tip vortexes (Q) to approach the following wings. As a result, there is also the potential for interference with the following wings.
Further, the leakage flows (S) merge together with the wing tip vortexes (Q), and the scale of the turbulence of the wing tip vortexes (Q) becomes larger and forms an even larger turbulence region near the outer peripheral portions of the rear edges 13b of the blade negative pressure surfaces 13e. 
Particularly in the case of a high static pressure type of fan where the axial direction length of the bellmouth 5 is large, these problems become more pronounced because it becomes easier for the scale of the turbulence resulting from the leakage flows (S) to become larger.
In other words, it is difficult to sufficiently control the turbulence and change in the trajectories of the wing tip vortexes (Q) that the above-described leakage flows (S) create with just the conventional bent portions 13c on the blade outer peripheral portions. Additionally, stably creating such wing tip vortexes (Q) near the negative pressure surfaces 13e has been insufficient, which has caused a rise in blowing noise.
The present invention has been devised in order to solve this problem, and it is an object thereof to provide an axial flow fan configured such that parts of the bent portions bent towards negative pressure surfaces of blade outer peripheries are further bent towards the negative pressure surfaces in regions near rear edges thereof, to thereby allow the aforementioned leakage flows to smoothly flow out downstream to reduce the vortex scale of the aforementioned leakage flows themselves and to be able to effectively control turbulence of the flows.